A deepthroatblowjob so deep one can see the bulge in her neck.
Normally, in order to produce this result, the woman must lie down on her back with her head off the side of the bed. The male approaches and from behind her head, such that his shaft is perfectly aligned with her throat.
*WARNING* If she chunders, prepare to roll her over or else she will drown and die. *WARNING*

I was just necking my bitch when I blew my load down her throat. She was so surprised, she chundered and I got an acid cock.

In Holden's Case, it means to "neck" with a female; kissing as some may say, or sexual activity. When Giraffes mate or "kiss" they have their butting of heads, a competition for the males to neck the other male away.

Whether it's meant by pleasing somebody or giraffes necking each other to get the female, it's all brutal.